why is atmosphere reentry such an issue? If it is just because of speed hitting the atmosphere why can’t the spacecraft just slow down before and synchronise with earth rotation?
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Your velocity in orbit is the thing keeping your craft in orbit. By slowing down you begin a change of your orbital parameters that leads to a lower and lower orbit, to the point that you begin reentry.
The entire idea of orbit is imagining a cannon capable of firing a ball so fast that its horizontal motion is so much more than its vertical descent that the curvature of the planet drops away faster (or at the same speed) as its ballistic arc.
You could, in theory halt your velocity suddenly and sharply, but at the speeds we’re talking about any tech aboard, or humans, would be crushed to paste by the acceleration.
Orbit isn’t just go up until the gravity goes to zero and you float, astronauts in low orbit experience ~90% the same force of gravity as we do on the ground. They don’t appear to “fall” because they move sideways so fast, that the Earth curves away from them at the same rate they fall – thus they are in constant free fall which feels like zero-g.
The velocities to do this are ludicrous, on the order of 8-9 kilometers per second. For context that means the ISS, a 400 thousand kilogram station, travels 10 times faster than a .50 bullet.
It takes so much fuel to get up to speed that if you wanted to slow down with retro-rockets you’d need the same amount of fuel you originally started with at launch, with you in orbit. However in order to get this fuel up to orbit, it requires even more fuel. The math works out that no spacecraft in history has ever even tried to bring fuel when theres this convenient atmosphere you can slam into to slow down. Rather that costing millions of pounds in fuel, it costs a few hundred of heat shield
To be fair I’m no expert but, if you slow down you fall down, because you lose orbital speed. You can’t slow down without losing height. If you synchronize with earth orbit its like standing still, which means you’re falling straight down
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